Topic: cr 26 carb on cb 550 / 605 ccm

[tobias helfert]

hello from germany, i am searching for a good setting for my cr 26 carb on a cb 550 with 605 ccm, cb 750 pistons, 4-1 exhaust. has anybody done that  and can hand over some setting informations ?
main jet 105, needle (p151) on clip 3 or 4 ( from upside ), slow jet 55, main air 220, airscrew 3/4 out.
i have problems getting on higher rpm. setting is well so far up to 1/2 throttle opening than an big hole , struggeling on same rpm ( nothing happens ) and than suddenly blasting.

thanks for any information
tobias

[davesprinkle]

Tobias, I am running the same carbs on my CB400F.  I struggled to solve this problem and finally realized that there is no solution.  I cannot open the throttle to 100% unless the engine speed is higher than 7000rpm.

[MRieck]

Tobias, I am running the same carbs on my CB400F.  I struggled to solve this problem and finally realized that there is no solution.  I cannot open the throttle to 100% unless the engine speed is higher than 7000rpm.

If you fill the bottom of the intake ports with epoxy it wll work

[tobias helfert]

hello mrieck,
do you think of the aluminum  intake ports ? has that something to do with the heat from cylinder head ? is the hot aluminum vaporizing the fuel-air mixture ? or does it at least becomes to less on its way past the valves to work properly with 600ccm ?
i had the feeling yesterday problems grew with high engine temperatur. i also had the feeling i got to less fuel to the engine at high rpm.
i will test with heatshield and several main air jets this week.
what is your setting, specially what needle diameter do you use ? i use  a p 151 with 5 clip positions. it is different from the standart y or yy.

tobias

[tobias helfert]

i forgot,  the "bottom of the intake ports " - you mean  in side the pipes or the bearing surface to the cylinderhead where the o-ring seal is positioned ?
tobias

[MaaseyRacer]

Tobias, I am running the same carbs on my CB400F.  I struggled to solve this problem and finally realized that there is no solution.  I cannot open the throttle to 100% unless the engine speed is higher than 7000rpm.

If you fill the bottom of the intake ports with epoxy it wll work

Does anyone have images of this?  I am just trying to get a better image of the amount of epoxy used.  These CR carbs are sounding like a wee bit of a pain in the rear.  However, I have a feeling that my CB350/400/466 hybrid is going to fly once they are dialed in.

[Chris Lee]

Hi Guys,

I'm from Australia and I have recently purchased the CR 26 Special carbs for my 78 550 as my old ones were pissing out fuel. Anyway I have bolted them on, bike starts nice, runs nice until I pull back on the throttle quickly and she struggles before catching up to herself and away it goes. Basically If I ride around gently it would be fine but who wants to do that. Could anyone advise me what my issue could be. No modifications to the bike that i'm aware of, just a 4 into 1 exhaust, cheers guys.

[Rod]

If your slow running is ok & acceleration occurs with gradual throttle opening but stalls with rapid throttle action, the main jet/needle combination is too rich or to lean - to distinguish which one you can use plug colour as a guide, back to back runs with needles on lowest & highest clip or easier still, deliberately create the stall with rapid opening and then roll off the throttle slightly, you will notice a slight pick up of power as you roll off if lean mixture is the problem (too lean also has a greater tendency to create backfire). 105 main follows the standard 5 to 1 formula for your carb size, but is only a starting point & the Y0 (151) needle is on the leaner side of the needle range - have you seen these? they're from another thread on the forum

http://www.satanicmechanic.de/download/Keihin%20Troubleshooting%20Guide.pdf

http://www.satanicmechanic.de/download/Keihin%20CR%20Special%20Owners%20Manual.pdf

[bwaller]

I think Mike may have been speaking about the 26mm carbs on the 400F. BUT we need to remember that tuning carbs that are "on the large side" becomes more difficult. Largecarbitis, I call it.

However those 26mm should work fine on the original posters 550 with 750 pistons. There are many members here who have done this. Perhaps also ask in the "projects" section where this has been discussed before.

Also on one point in Rod's good advice, I think you'll find if the throttle is cracked wide open, and stutters, then on closing the throttle it picks up is usually is a sign of over richness. Closing the slide drops the needle (into the needle jet) onto a thicker portion of the taper leaning the mixture. 

[Tintop]

Agree with Brent that this is a common symptom of as he calls 'large carbitis'.  Basically it is an overly lean condition caused flat spot.  The issue is the sudden increase in the amount of air vs fuel.  The carb needs air speed to draw more fuel, and that isn't happening at that instant.  You either need to speed up the air (what Mike's epoxy suggestion does), or instantly add more fuel to cover the lean condition (what an acccel. pump does).  Why I love Webers.

[Rod]

Also on one point in Rod's good advice, I think you'll find if the throttle is cracked wide open, and stutters, then on closing the throttle it picks up is usually is a sign of over richness. Closing the slide drops the needle (into the needle jet) onto a thicker portion of the taper leaning the mixture.

Sorry, I should have been more specific Brent - if you roll off a wide open throttle slightly and I mean just slightly, there will be a momentary drop in pressure in the venturi because air on the inlet side still posses the velocity & mass (momentum) created by the wide open throttle but now has to increase velocity in order to get through the (now) more restricted venturi, creating a greater pressure drop & therefore (momentarily) more fuel being delivered for the given mass of air, this tends to more than offset the effect of the needle having moved down a little on the taper and the consequent (brief) richening of the mixture wouldn't normally be noticeable except where a setup is unduly lean. All that said, this is just my imperfect understanding of the phenomenon, but is was explained to me many years ago by a brilliant (but largely unknown) head tuner called Les Ryder who worked in the Le Mans winning Jaguar team which dominated for a number of years in the 50's, he then went on to collaborate with Vincent Piatti on the ground-breaking twin-spin chambers. But as I said, I'm no fluid dynamics expert so shoot me down if I'm wrong.